It can be used however to support the idea that the Knight and Lady can have sex. True, there is still the argument that can be made against it. Now, if she in fact meant "supposed to" knowing it is in fact an occurrence, then it comes that much closer to a lie. So, I agree with your statement that it isn't a lie, and it largely is an opinion but it doesn't preclude it from being a fact that the knight and lady can have sex. They definitely in Maeve's opinion should want to.
In any event, the point-at least in another thread-was that Maeve was trying to co-opt Harry for her purposes. Most likely to aid her in freeing the Sleepers prior to banefire. I forget to which purpose I place certain posts under specific threads. This one was the knight have sex with lady idea.